IUG Journal of Natural Studies Peer-reviewed Journal of Islamic University-Gaza ISSN 2409-4587 IUGNES Vol. 25, No 2, 2017, pp 128-134 128 IUG Journal of Natural Studies (Islamic University of Gaza) / CC BY 4.0 Accepted on (14-03-2017) 1. Introduction: It is well known that the pore structure of hardened cement pastes strongly influence their physical properties. The presence of an air- entraining agent (AEA) not only improves workability, but also enhances its freeze-thaw resistance (Lea's, 2004). Du and Folliard (2005) noted that there is a minimum dosage of AEA required to entrain air in the concrete. Rosen (2004) reported that the formed air bubbles in cement pastes are unstable and have limited lifetimes. Therefore, surfactants as AEAs are used in many cases to entrain air bubbles and stabilize them in the fresh cement pastes. Lea (1971) and Zhang (2001) suggested that air entraining surfactants operate at the air-water and solid-water interfaces (Figure 1). They help reducing surface tension of water, thus promoting dispersion and bubble forming. Leslie and Qingye (2004) concluded that the addition of AEA increased the air content up to a saturation level, above which no further increase in air content was observed. Qaraman (2016) and Algurnon (2013) stated that a number of factors influences entrained air. Examples of such as factors include the duration of mixing and the nature and concentration of the surfactants used as air-entraining agents. Concerning the mixing time, Patrick (1968) reported that duration of stirring affects the air void system in such a way as to cause an increase in the air content and the specific surface area of the voids as the mixing time increases. In addition, Yang (2012) observed that mixing a longer time could entrain more air in fresh cement pastes by applying more work on it. The Mode of Action of LM and SLBS Surfactants on the Properties of Cement Pastes Abdel Fattah A. Qaraman 1,* , Shafik M. Jendia 2 , Wafaa S. Hegaz 3 , and Faten Z. Mahmoud 3 1 Department of Scientific Research, Israa University, Gaza Strip, Palestine 2 Department of Civil Engineering, Faculty of Engineering, Islamic University of Gaza, Gaza Strip, Palestine 3 Department of Chemistry, Women College, Ain Shams University, Cairo, Egypt * Corresponding author e-mail address: fatahdeep@gmail.com The Physiochemical and Mechanical Properties of cement pastes containing synthetic surfactants as air entraining agents are investigated by adding different percentages of each of the nonionic surfactant Luramide 11 (LM) and the anionic surfactant Sodium lauryl benzene sulfonate (SLBS) to some Portland cement pastes. The influence of the different surfactants concentrations and the mixing times on the air content of the pastes and accordingly the bulk density, compressive strength and microstructure of the hardened cement specimens is discussed. The optimal level of factors to achieve the desired goals is determined. The results show that LM is more preferable than SLBS in most cases. Keywords: Air-entraining admixtures, Density, Compressive strength, XRD, Luramide 11, Sodium lauryl benzene sulfonate, surfactant.